American presidential elections are increasingly indistinguishable from the reality TV competitions drowning the nation's airwaves. Both are vapid, personality-driven and painfully protracted affairs, with the winners crowned by virtue of their ability to appear slightly more tolerable than the cast of annoying rejects whom the public eliminates one by one. When, earlier this year, America's tawdriest (and one of its most-watched) reality TV show hosts, Donald Trump, inserted himself into the campaign circus as a threatened contestant, he fitted right in, immediately catapulting to the top of audience polls before announcing he would not join the show.

The Republican presidential primaries – shortly to determine who will be the finalist to face off, and likely lose, against Barack Obama next November – has been a particularly base spectacle. That the contest has devolved into an embarrassing clown show has many causes, beginning with the fact that GOP voters loathe Mitt Romney, their belief-free, anointed-by-Wall-Street frontrunner who clearly has the best chance of defeating the president.

In a desperate attempt to find someone less slithery and soulless (not to mention less Mormon), party members have lurched manically from one ludicrous candidate to the next, only to watch in horror as each wilted the moment they were subjected to scrutiny. Incessant pleas to the party's ostensibly more respectable conservatives to enter the race have been repeatedly rebuffed. Now, only Romney remains viable. Republican voters are thus slowly resigning themselves to marching behind a vacant, supremely malleable technocrat whom they plainly detest.

In fairness to the much-maligned GOP field, they face a formidable hurdle: how to credibly attack Obama when he has adopted so many of their party's defining beliefs. Depicting the other party's president as a radical menace is one of the chief requirements for a candidate seeking to convince his party to crown him as the chosen challenger. Because Obama has governed as a centrist Republican, these GOP candidates are able to attack him as a leftist radical only by moving so far to the right in their rhetoric and policy prescriptions that they fall over the cliff of mainstream acceptability, or even basic sanity.

In July, the nation's most influential progressive domestic policy pundit, New York Times columnist Paul Krugman, declared that Obama is a "moderate conservative in practical terms". Last October, he wrote that "progressives who had their hearts set on Obama were engaged in a huge act of self-delusion", because the president – "once you get past the soaring rhetoric" – has "largely accepted the conservative storyline".

Krugman also pointed out that even the policy Democratic loyalists point to as proof of the president's progressive bona fides – his healthcare plan, which mandates the purchase of policies from the private health insurance industry – was designed by the Heritage Foundation, one of the nation's most rightwing thinktanks, and was advocated by conservative ideologues for many years (it also happens to be the same plan Romney implemented when he was governor of Massachusetts and which Newt Gingrich once promoted, underscoring the difficulty for the GOP in drawing real contrasts with Obama).

How do you scorn a president as a far-left socialist when he has stuffed his administration with Wall Street executives, had his last campaign funded by them, governed as a "centrist Republican", and presided over booming corporate profits even while the rest of the nation suffered economically?

But as slim as the pickings are for GOP candidates on the domestic policy front, at least there are some actual differences in that realm. The president's 2009 stimulus spending and Wall Street "reform" package – tepid and inadequate though they were – are genuinely at odds with rightwing dogma, as are Obama's progressive (albeit inconsistent) positions on social issues, such as equality for gay people and protecting a woman's right to choose. And the supreme court, perpetually plagued by a 5-4 partisan split, would be significantly affected by the outcome of the 2012 election.

It is in the realm of foreign policy, terrorism and civil liberties where Republicans encounter an insurmountable roadblock. A staple of GOP politics has long been to accuse Democratic presidents of coddling America's enemies (both real and imagined), being afraid to use violence, and subordinating US security to international bodies and leftwing conceptions of civil liberties.

But how can a GOP candidate invoke this time-tested caricature when Obama has embraced the vast bulk of George Bush's terrorism policies; waged a war against government whistleblowers as part of a campaign of obsessive secrecy; led efforts to overturn a global ban on cluster bombs; extinguished the lives not only of accused terrorists but of huge numbers of innocent civilians with cluster bombs and drones in Muslim countries; engineered a covert war against Iran; tried to extend the Iraq war; ignored Congress and the constitution to prosecute an unauthorised war in Libya; adopted the defining Bush/Cheney policy of indefinite detention without trial for accused terrorists; and even claimed and exercised the power to assassinate US citizens far from any battlefield and without due process?

Reflecting this difficulty for the GOP field is the fact that former Bush officials, including Dick Cheney, have taken to lavishing Obama with public praise for continuing his predecessor's once-controversial terrorism polices. In the last GOP foreign policy debate, the leading candidates found themselves issuing recommendations on the most contentious foreign policy question (Iran) that perfectly tracked what Obama is already doing, while issuing ringing endorsements of the president when asked about one of his most controversial civil liberties assaults (the due-process-free assassination of the American-Yemeni cleric Anwar Awlaki). Indeed, when it comes to the foreign policy and civil liberties values Democrats spent the Bush years claiming to defend, the only candidate in either party now touting them is the libertarian Ron Paul, who vehemently condemns Obama's policies of drone killings without oversight, covert wars, whistleblower persecutions, and civil liberties assaults in the name of terrorism.

In sum, how do you demonise Obama as a terrorist-loving secret Muslim intent on empowering US enemies when he has adopted, and in some cases extended, what was rightwing orthodoxy for the last decade? The core problem for GOP challengers is that they cannot be respectable Republicans because, as Krugman pointed out, Obama has that position occupied. They are forced to move so far to the right that they render themselves inherently absurd.